This invention relates to an AC capacitor containing a dielectric fluid and 2 to 10% .alpha.-naphthyl isocyanate as additive to improve the capacitance retention and hence life characteristics of the capacitor. Virtually all dielectric fluids will dissolve trace amounts of water, and the silicone dielectric fluids are no exception. In the case of the silicone fluids, however, traces of water have a much more severe and adverse impact than that found with hydrocarbon or ester dielectric fluids. The insulation resistance is greatly reduced and the electrical properties are vitiated.
Water pickup during impregnation can be avoided by controlling the plant atmosphere and drying the oil. However, since AC film capacitor cases and seals are not truly hermetic, trace amounts of water and somewhat larger amounts of air containing oxygen will enter the capacitor and hence the fluid, impairing and degrading it.
Under voltage, aluminum metallization can react with water vapor or oxygen, converting the aluminum to the oxide, an insulator, with a resultant extensive capacity loss. By a mechanism which is not now fully understood the dielectric system of the present invention is effective in preventing this capacity loss. This effect may result either from the material functioning as a water scavenger or as an oxygen inhibitor.